Not applicable.
The present invention is directed to a method for producing a material from a metallurgical powder and to the material produced by the method. More particularly, the present invention is directed to a method for producing a material from a metallurgical powder including iron, copper, and graphite, and wherein the method generally includes providing a sintered compact of the powder, densifying at least a portion of the compact, and subsequently increasing the surface hardness of the compact to greater than RC 25, and preferably at least RC 30. Material produced by the method exhibits high rolling contact endurance limit and/or high tensile strength. Specific examples of applications in which the method and material may be applied include races, gears, sprockets, and cam lobes.
The xe2x80x9csinterhardeningxe2x80x9d process is a known process in which iron-based alloys having high hardness are produced by consolidating and sintering metallurgical powders. The alloying element and carbon contents of the metallurgical powder and the cooling rate of the sintered parts within the sintering furnace are carefully balanced to produce parts having a surface hardness greater than about Rockwell C (RC) 25 directly from the sintering furnace, without the requirement for a conventional quench-and-temper treatment. Parts having surface hardness greater than RC 25 are typically produced by sinterhardening using a furnace that is specially designed to gas cool the hot sintered parts at an accelerated rate, in the 120-200xc2x0 F./minute range. More recently, sinterhardening processes have been designed to utilize metallurgical powders with higher alloy contents that can be hardened to greater than RC 25 on cooling using conventional sintering furnaces providing standard cooling rates, typically about 40xc2x0 F./minute.
Sinterhardened parts are normally hard and strong (tensile strengths in the 120 to 160 ksi range). A primary advantage of the sinterhardening process is that the conventional quench-and-temper cycle is unneeded, reducing the number of processing steps and reducing the cost of finished parts. A second advantage is that gas cooling is less severe and causes less warpage than liquid cooling. Because sinterhardened parts are gas cooled rather than liquid cooled, there is generally less dimensional distortion in the parts and size control is enhanced. In addition, because there is no need to dispose of an oil or other liquid quenching medium, the impact on the environment is lessened.
A distinct shortcoming of parts produced by sinterhardening is relatively low rolling contact endurance limits, usually in the 160 to 190 ksi range. The rolling contact endurance limit, also referred to herein plainly as the xe2x80x9cendurance limit,xe2x80x9d is the theoretical maximum stress that a material can withstand for an infinitely large number of compressive fatigue cycles. The endurance limit of a material may be assessed by, for example, the method described in U.S. Pat. No. 5,613,180, the entire disclosure of which is hereby incorporated herein by reference. The testing method generally described in the ""180 patent was used to measure the endurance limit of the materials described herein.
Rolling contact endurance is particularly important in powder metal parts such as races, gears, sprockets, and cam lobes. The relatively low rolling contact endurance limit of sinterhardened materials is not entirely unexpected because the endurance limit is strongly dependent on material density. Denser materials typically have higher endurance limits. Parts produced by sinterhardening commonly have apparent densities of about 7.0 g/cc or less, which may be compared with typical theoretical densities of about 7.9 g/cc for sinterhardening alloys.
Materials produced by sinterhardening may have tensile strength significantly greater than powder metal materials of comparable density produced by conventional quench-and-temper techniques. Tensile strength of sinterhardened parts typically falls in the range of 130 to 150 ksi. This may be compared with the 100-110 ksi tensile strength of conventional quenched and tempered powder metal material at 7.0 g/cc. Conventional materials, because they are based on xe2x80x9csofterxe2x80x9d powders and do not harden on sintering at 1400-1600xc2x0 F., can be double processed to densities in the 7.3-7.5 g/cc range. Increasing part density can provide increased tensile strength, and also can increase endurance limit. Heat treated double pressed/double sintered parts, for example, can achieve heat treated tensile strengths of 160-200 ksi. The higher tensile strength that may result from increased density may be desirable in parts used as races, gears, sprockets, cam lobes, connecting rods, and in other high load-bearing applications. Such applications usually also require high endurance limit. In contrast, increasing the density of sinterhardened parts to provide higher endurance limits and tensile strength is problematic. The metallurgical powder grades used in sinterhardening are highly alloyed and, therefore, are not highly compressible. Also, because sinterhardened parts emerge from the sintering furnace relatively hard, they are not easily densified by mechanical working techniques such as sizing. Cost and other benefits derived by avoiding a quench-and-temper cycle are, in part, offset by the difficulties faced when densifying sinterhardened parts. Thus, although the sinterhardening process provides distinct advantages, it is not widely used to produce powder metal parts for the heaviest duty races, gears, sprockets, and cam lobes, applications requiring high rolling contact endurance limits and/or high tensile strength.
Accordingly, a need exists for a process for producing parts from consolidated metallurgical powder wherein the parts are of high density and surface hardness greater than about RC 25, without the need for a conventional liquid quench-and-temper treatment. A need also exists for a process for producing powder metal parts having high rolling contact endurance limits and/or high tensile strength, and wherein the parts are surface hardened to greater than RC 25 without the need for a conventional liquid quench-and-temper treatment.
In order to address the above-described needs, the present invention provides a novel method for producing a material from a metallurgical powder. The method includes providing a metallurgical powder that includes iron, 1.0 to 3.5 weight percent copper, and 0.3 to 0.8 weight percent carbon. The carbon in the metallurgical powder preferably is wholly or predominantly in the form of graphite. The copper in the metallurgical powder preferably is wholly or predominantly in the form of elemental copper powder. The metallurgical powder also may include, for example, nickel, molybdenum, chromium, manganese, and vanadium. The metallurgical powder preferably includes molybdenum and/or nickel in the form of a pre-alloyed iron-base powder.
At least a portion of the metallurgical powder is compressed at a pressure of 20 tsi to 70 tsi to provide a compact. The compact is heated to a temperature of 2000-2400xc2x0 F. and is maintained at the temperature for at least 15 minutes. The heated compact is then cooled at a cooling rate no greater than 60xc2x0 F./minute. The rate of cooling is selected so that the compact, once cooled, has hardness no greater than RC 25, and preferably no greater than RC 20. Subsequent to cooling the compact, the density of at least a surface region of the compact is increased to at least 7.6 grams/cc. The density of the compact may be increased by, for example, mechanically working the sintered compact. The mechanical working technique that is used may be one or more of, for example, sizing, rolling, roller burnishing, shot peening, extruding, laser impacting, swaging, and hot forming. The densification technique may be applied to increase the density of a surface region or some other region of the compact, but also may be applied to increase the density throughout the compact. The densified compact is then heated to a temperature of 2050-2400xc2x0 F. and held at temperature for at least 20 minutes. The heated compact is cooled at a cooling rate greater than the rate of the first cooling step and within the range of 120-400xc2x0 F./minute so as to increase surface hardness of the compact to greater than RC 25, and preferably at least as great as RC 30.
The present invention also is directed to a method for producing a material from a metallurgical powder, as the powder is described immediately above, and wherein at least a portion of the powder is compressed at a pressure of 20 tsi to 70 tsi to provide a compact. The compact is processed by heating and then cooling the compact. The apparent density of the cooled sintered compact is 6.2 to 7.2 grams/cc. The cooling rate is no greater than about 60xc2x0 F. per minute so that the surface hardness of the cooled sintered compact increases to no greater than RC 25. The density of at least a portion of the sintered compact is then increased to at least 7.6 grams/cc, and the densified compact is then heated to provide a heated sintered compact. The heated sintered compact is cooled at a rate sufficient to increase the surface hardness of the compact to greater than RC 25, and preferably at least RC 30.
As noted above, carbon may be wholly or partially present in the metallurgical powder as graphite in either of the above methods. Carbon may also be present in the metallurgical powder in other forms, such as in the form of carbon alloyed with other elements as pre-alloyed powders. The carbon content, copper content, and the content of the other elements present in the metallurgical powder are selected so that on heating and then slowly cooling a compact of the powder, the hardness of the compact does not exceed RC 25.
Additional aspects of the present invention are directed to materials produced by the method of the invention and articles of manufacture including such materials. The articles of manufacture may be, for example, races, gears, sprockets, and cam lobes.
The surface hardness of materials provided in the present description are referred to by several different hardness scales, including RC, Rockwell B (RB), and 15N hardness. Each hardness scale used herein is the resistance to indentation as measured by a Rockwell hardness tester or a microhardness tester. Both tester types operate by forcing an indenter of a specified geometry and material into the surface of a test specimen under a controlled force, and the depth of penetration is measured. The hardness scale used to measure a particular part normally is tied to the application of that part. Those of ordinary skill in the art may readily convert an apparent hardness of one hardness scale (for example, RC, RB, or 15N) to another scale. The specific techniques by which hardness may be evaluated under any of the scales used herein also will be readily apparent to those of ordinary skill.
Material may be produced by the process of the invention with high surface hardness, greater than RC 25. The material also may have a relatively high endurance limit, at least about 240 ksi, and high torque and/or tensile strength. In the initial steps of the method of the invention, a readily deformable compact is produced that may be further densified. The compact is then densified in part or throughout to provide one or more highly dense regions, thereby providing high rolling contact endurance limit. Thus, the difficulties encountered in attempting to densify sinterhardened materials, which typically have surface hardness in excess of RC 25, are avoided. A sinter followed by an accelerated cooling step, which preferably is a gas cooling step, increases the surface hardness of the material to greater than RC 25, preferably at least RC 30, hardness levels commensurate with or superior to conventional sinterhardened materials. Utilizing gas cooling in the accelerated cooling step avoids the dimensional control difficulties encountered with conventional liquid quench-and-temper treatments. In addition, gas cooling does not require a liquid quenching media that must be disposed of as waste. Thus, the method of the invention provides a material with properties superior to conventional sinterhardened materials, yet also providing processing advantages garnered by the sinterhardening process.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend additional advantages and details of the present invention upon carrying out or using the invention.